Magnetic chuck control system

ABSTRACT

Prior art magnetic chuck control systems do not utilize state of the art components and do not sufficiently isolate the power output from control circuits. The instant invention discloses a magnetic chuck control system employing solid state circuitry including a power output circuit (24) adapted to apply a DC voltage to an object such as a magnetic chuck (12) or the like so as to magnetize the same. The power output circuit employs a triac reversing circuit and is interfaced with a digital control circuit (26) and a digital-to-analog converter circuit (30) so as to apply alternating polarity voltage signals of successively reduced magnitude to the chuck during a demagnetizing operation. All high voltage is confined to the power output circuit so as to provide significant safety protection to the operator. Various control functions such as full power, variable power, residual power and release have digital interlocking so that an operator must effect full power or variable power after a residual power function.

This application is a continuation-in-part of Ser. No. 101,508, now U.S.Pat. No. 4,306,269, filed Dec. 7, 1979.

The present invention relates generally to magnetic control systems, andmore particularly, to a magnetic control system employing novel solidstate circuit means for effecting demagnetizing of an object such as amagnetic chuck and associated ferromagnetic work piece or the like.

It is known that in employing a magnetic chuck to hold a ferromagneticwork piece in a machine tool or the like, the chuck and the work piece,upon deenergizing, retain significant residual magnetism which inhibitsrelease of the work piece from the chuck. It is conventional in suchcases to demagnetize the magnetic chuck and associated work piece so asto enable release of the work piece. Such demagnetizing is generallyaccomplished by passing a current through the chuck, and thereby theassociated work piece, in alternating opposite directions by reversingthe polarity in a series of steps while lowering the voltage potentialin each successive step until the voltage potential reachessubstantially zero at which time the chuck and work piece aresufficiently demagnetized to allow the work piece to be readilyseparated from the magnetic chuck.

A general object of the present invention is to provide a novel magneticcontrol system which finds particular application in controllingmagnetic chucks and the like, and which is adapted to effectdemagnetizing of the chuck and associated work piece so as to enable anoperator to readily remove the work piece from the chuck.

A more particular object of the present invention is to provide atotally solid state magnetic chuck control system employing a poweroutput circuit interfaced with a digital control circuit anddigital-to-analog converter circuit so that the power output circuit isadapted to apply output voltage signals to the chuck of successivelydecreasing magnitude and alternating polarity during a demagnetizingoperation.

Still another object of the present invention is to provide a magneticchuck control system having a power output circuit connected to a DCpower supply and interfaced with a digital control circuit and adigital-to-analog converter circuit operative on the power outputcircuit to establish output voltage signals to the chuck of alternatingpolarity and successively decreasing magnitude during a demagnitizingoperation, the digital control and digital-to-analog circuit being oflow voltage operation and isolated from the high voltage power outputcircuit so as to protect the operator should a fault current occur inthe power output circuit.

A feature of the magnetic chuck control system in accordance with thepresent invention lies in the provision of a visual display adapted todigitally indicate the various cycles of voltage reversal during ademagnetizing operation and thereby inform the operator whendemagnetizing has been completed.

Another feature of the magnetic chuck control system in accordance withthe present invention lies in the employment of a digital-to-analogconverter circuit for controlling the magnitude of the output signalsfrom the power output circuit to the magnetic chuck, thedigital-to-analog converter circuit including a variable poweradjustment enabling selection of the magnetic chuck holding power to amagnitude less than 100% of full power.

Another feature of the magnetic chuck control system in accordance withthe present invention lies in the use of triacs in a reversing circuitportion of the power output circuit, the triacs being connected in abridge type circuit and having their respective gates interfaced withthe digital control circuit through optical couplings so as to isolatethe digital control circuit from the substantially higher currentcontrolled by the triacs.

Still another feature of the magnetic chuck control system in accordancewith the invention lies in the provision of various control functionssuch as full power, variable power, residual power and release, andwherein the various functions are digitally interlocked so that anoperator must effect full or variable power after a residual powerfunction to insure complete reduction of any residual magnetism to zero.

Further objects and advantages of the present invention, together withthe organization and manner of operation thereof, will become apparentfrom the following detailed description of the invention when taken inconjunction with the accompanying drawings wherein like referencenumerals designate like elements throughout the several views, andwherein:

FIG. 1 is a perspective view of a control console containing the variouscircuits comprising the magnetic chuck control system in accordance withthe present invention;

FIG. 2 is a schematic block diagram of the various circuits comprisingone embodiment of the magnetic chuck control system of the invention;

FIG. 3 is a circuit diagram of the power supply circuit, the optional ACon-off switching circuit and the DC power output circuit of the magneticchuck control system illustrated schematically in FIG. 2;

FIG. 4 is a circuit diagram of the digital control and digital-to-analogconverter circuits of the magnetic chuck control system illustratedschematically in FIG. 2.

FIG. 5 is a logic diagram illustrating the various logic states of themagnetic chuck in relation to the digital control circuitry; and

FIG. 6 is a schematic diagram showing the relation of the various logicstates of the optical couplers for controlling current flow through themagnetic chuck in relation to the logic states of the associated controlcircuitry.

The various features of the magnetic control system in accordance withthe present invention are described herein, by way of illustration, inconjunction with a control system, indicated generally by referencenumeral 10 in FIG. 2, for controlling a magnetic chuck, indicatedschematically at 12 in FIGS. 2 and 3. The magnetic chuck 12, which mayalso be termed an electromagnet, is adapted to have a ferromagnetic workpiece or other object, such as indicated in phantom at 14, mountedthereon and magnetically maintained in fixed relation on the chuckduring the performance of various operations on the work piece. Forexample, the magnetic chuck 12 may be employed in a machine tool as awork holder during which various operations may be performed on the workpiece. The magnetic chuck 12 may, in turn, be mounted on a reciprocatingtable or the like, so that the associated work piece 14 is moved inpredetermined relation to a tool, or alternatively, the magnetic chuckand associated work piece may be maintained in stationary relation andthe tool moved relative to the chuck. Magnetic chucks of the type withwhich the control system of the present invention may be employed arewell known and do not, per se, form a part of the present invention.

Very generally, the magnetic control system 10 includes an AC on-offswitchinq circuit 20 which, in the described embodiment, is optional butis shown connected to a suitable AC power supply, such as a 115 VACsupply, and is also connected to a power supply circuit 22 in a mannerto enable energizing of the power supply circuit by the 115 VAC powersupply. The power supply circuit 22 includes a bridge rectifier toprovide a 115 VDC output which is applied to a DC power output circuit24 the output of which is connected to the magnetic chuck 12 tofacilitate magnetic holding of a ferromagnetic work piece or object 14on the magnetic chuck. The power output circuit 24 is interfaced with adigital control circuit 26 and a digital-to-analog converter circuit 30which cooperate to enable magnetizing and demagnetizing of the chuck andassociated work piece. As will be described more fully hereinbelow, theconverter circuit 30 includes a variable power adjustment which enablesselection of the magnetic chuck holding power to a magnitude less than100% of full power.

The power supply circuit 22 is also adapted to provide regulated lowvoltage DC output signals, such as ±5 VDC, to the digital controlcircuit 26 which establishes digital output pulse signals ofpredetermined equal interval timed relation and which are counted anddisplayed on a visual display 28 adapted to provide a digital indicationto the operator of the polarity reversal cycle and the polarity of thesignal then being applied to the chuck during a demagnetizing operation.

The timed digital pulse signals from the digital control circuit 26 arealso applied to the digital-to-analog converter circuit 30 which alsoreceives a +5 VDC power supply from the power supply circuit 22. Thedigital-to-analog converter circuit 30 is adapted to supply an amplifiedanalog output signal to the power supply circuit 22 and cooperates withthe digital control circuit 26 to control the AC power applied to thebridge rectifier of the power supply circuit and thus the DC powerapplied to the magnetic chuck.

The DC power output circuit 24 includes a reversing circuit in the formof triacs connected in a bridge type network and having their gatestriggered by the digital control circuit 26 so as to provide precisedigital timing. The reversing circuit controls the direction of currentflow through the magnetic chuck 12 and thus the polarity of the voltageoutput signals applied to the magnetic chuck 12 during a demagnetizingoperation. The successive voltage signals applied to the chuck 12 duringdemagnetizing are of precise predetermined decreasing magnitude asestablished by the digital-to-analog converter circuit 30.

The various circuits 20, 22, 24, 26 and 30 of the magnetic chuck controlsystem 10 are preferably contained within a suitable waterproof consoleor housing such as indicated generally at 34 in FIG. 1. In theillustrated embodiment, the console 34 comprises a generally rectangularhousing which is substantially closed on all sides and has a front wall36 forming a removable control panel on which are mounted the digitaldisplay 28 and various switches and associated function indicator lampsas will be described. Suitable tubular conduits 40a and 40b areconnected to the housing 34 for a 3-wire AC power supply and forconductors connecting the DC power output circuit 24 to the magneticchuck 12.

Referring now to FIGS. 3 and 4 for a more detailed description of thevarious circuits comprising the magnetic chuck control svstem 10 asillustrated schematically in FIG. 2, the AC on-off switching circuit 20includes a dual voltage control transformer 46 having a fused primaryconnected to the 115 VAC power source through a suitable fuse 48.Transformer 46 has a low voltage secondary adapted to provide two 15 VACoutputs which are connected in circuit with suitable diodes 50a and 50b,an input filter capacitor 52, a voltage regulator 54 and an outputfilter capacitor 56 to provide a regulated 5 VDC for a light emittingdiode (LED) 58, an LED 60, an optically isolated triac driver 62,alternatively termed an optical coupler, and a pair of seriallyconnected NAND gates 64a and 64b. The NAND gates 64a and 64b form abistable switch having an output at pin 6 which can assume logic states"0" or "1". Logic "0" is ground and "1" is approximately 5 VDC. Pin 2 ofNAND gate 64b is "1" through a current limiting resistor 66a to (+) 5VDC, and pin 5 of NAND gate 64a is "1" through a current limitingresistor 66b to (+) 5 VDC. This insures pins 2 and 5 being held to a "1"state at the first moment of applying power to the circuit.

As aforementioned, the AC on-off switching circuit 20 is optional. It iscontemplated that the switching circuit 20 be employed for large sizemagnetic chuck control systems, such as for greater than approximately1000 watt control systems. With magnetic chuck control systems forsmaller power requirements, such as below 1000 watts, the switchingcircuit 20 may optionally be eliminated and the AC power supply for thepower supply circuit 22 obtained from the machine with which the system10 is used or another suitable source.

If the on-off switching circuit 20 is employed, the LED 58, which maycomprise an individual lamp of T-1 3/4 size adapted to give off a redlight when energized, may be mounted on the panel 36 of the console 34.Similarly, LED 60, which may comprise a similar size lamp adapted togive off an orange light, may also be mounted on the control panel 36.When first applying power to transformer 46, pin 6 of NAND gate 64a isat a "1" state, which enables LED 60 to light. At this time, pin 3 ofNAND gate 64b is at a "0" state making pins 9 and 10 of an inverter 67a"0" and making pin 8 "1". This keeps LED 58 OFF. At this time pins 12 &13 of an inverter 67b are "0" and pin 11 is "1". This keeps opticalcoupler 62 off.

An AC normally open "on " switch 68, which preferably comprises a sealedmembrane type switch such as commercially available from SheldahlCorporation and which may be mounted on control panel 36, is connectedbetween ground and the NAND gate 64b so that closing switch 68, termedthe AC "on" switch when the switching circuit 20 is employed,establishes a "0" level at pin 6 of NAND gate 64a. This makes LED 60 gooff and turns on LED 58. Closing switch 68 also enables the opticalcoupler 62 to turn on so as to energize a triac 70, which, assumingconnection of the switch circuit 20 to the power supply circuit 22,enables the primary of a main power transformer 72 in the power supplycircuit 22 to be energized by the 115 VAC power source through a fuse74.

A switch 76, which also may be mounted on control panel 36 and maycomprise a normally open preferably sealed membrane type switch similarto switch 68, is connected between ground and NAND gate 64a so thatclosing switch 76 establishes a "1" level at pin 6 of NAND gate 64a.This turns off the optical coupler 62 and LED 58 and turns on LED 60.Turning off optical coupler 62 deenergizes the triac 70 and removes theAC voltage from the primary of the transformer 72. A capacitor 78 and aresistor 80 are connected across the triac 70 so as to form a phaseshift network to enable triac 70 to turn on and off.

An AC on-off switch 82 is connected in circuit with the primary oftransformer 72 for use when the power supply circuit 22 is connected toa 115 VAC supply derived from the machine with which the control system10 is employed or another suitable source, and the AC switching circuit20 is not utilized. The switch 82 is preferably mounted on the controlpanel 36 as illustrated in FIG. 1.

With the primary of power transformer 72 turned on, three separateregulated +5 VDC power supplies are derived from three separate lowvoltage secondaries of transformer 72, bridge rectifiers 84a, 84b and84c, input filter capacitors 86a, b and c, voltage regulators 88a, b andc, and output filter capacitors 90a, b & c. One regulated +5 VDC,designated "VCC", provides a low voltage power supply to the digitalcontrol circuit 26 and the digital-to-analog converter circuit 30 asshown by corresponding designations on the circuit diagrams of FIGS. 3and 4. The power supplies represented by voltage regulators VR2 and VR3are used in conjunction with the polarity reversing circuit in the DCpower supply circuit 22, as represented by (+) or (-) VR2 or VR3.

The power supply circuit 22 also includes a power bridge rectifier 92which is connected in circuit with parallel 130 VAC secondaries of thepower transformer 72 and provides a DC Output to the DC power outputcircuit 24. The DC power output of bridge rectifier 92 is regulated bycontrolling the AC power supply to the bridge rectifier through thediqital control circuit 26 and the digital-to-analog circuit 30 in amanner to be described. The DC power output circuit 24 is connected tothe magnetic chuck 12 through conductors 96a and 96b, alternativelytermed the magnetic chuck terminals, which are connected to a set ofoutput terminals 98a,b of a reversing circuit, indicated generally at100, which is in the form of a bridge type network and has a set ofinput terminals 102a,b connected, respectively, to the plus and minusterminals of the bridge rectifier 92.

The reversing circuit 100 includes four triacs 104a, b, c and d each ofwhich is connected in a leg of the bridge type network reversingcircuit. Each triac 104a, b, c and d has its gate operatively coupled tothe digital control circuit 26 through an associated optical coupler106a, b, c and d, respectively, which has a transistor output and servesas an optically isolated triac driver to enable selective switching ofthe corresponding triac into conducting and nonconducting states throughthe application of a relatively low voltage signal to the associatedtriac gate. The optical couplers 106a-d have very high resistance tocurrent flow in a direction toward the digital control circuit 26 so asto isolate and protect the digital control circuit from any power surgesor erratic signals in the power output circuit 24. In this manner, allhigh voltages are confined to the output circuit 24 and the highestpotential to which the digital control circuit 26 and display 28 may besubjected, and also switching circuit 20 and converter circuit 30, isthe low +5 VDC power supply from supply circuit 22. The triacs 104a, b,c and d define bidirectional current control means selectively connectedbetween the output terminals 98a,b and the input terminals 102a,b ofreversing circuit 100 so as to enable selective directional current flowthrough the magnetic chuck 12 by providing means for alternatelyswitching the polarity of the voltage signals applied to the terminalconnectors 96a,b of the magnetic chuck. The triacs 104a,b,c and d alsoserve to absorb voltage spikes generated by the magnetic chuck wheneverthe polarity across it is switched.

The magnetic chuck control system 10 is adapted for four principleoperating functions: full power, variable power, residual power andrelease. The full power function enables application of approximately115 VDC to the magnetic chuck 12. The variable power function enablesselection and application of a magnetic holding power less than 100% offull power. The residual power function enables retention of work pieceson the chuck 112 through a residual holding power or magnetism after themain power has been disconnected. This is desirable to allow an operatorto remove a work piece from the chuck, gage the work piece, and place itback on the chuck without disturbing other work pieces on the chuck. Therelease function serves to demagnetize any work piece on the chuck so asto allow it to be readily removed. As will be hereinafter described,control switches for selecting the various operating functions andassociated indicator lights for visually indicating which operatingfunction is energized are mounted on the control panel 36 for operatoraccess and observation. The control switches are connected in thedigital control circuit 26 in which the full power switch is indicatedat 110, the variable power switch is indicated at 112, the residualpower switch is indicated at 114 and the release switch is indicated at116. The switches 110, 112, 114 and 116 are preferably of the sealedmembrane type and may be mounted on the panel 36 as shown in FIG. 1.Corresponding function indicator lamps, preferably of the LED type, areshown at 118, 120, 122 and 124.

In accordance with one feature of the magnetic control system 10, thevarious operating function switches are electrically digitallyinterlocked so that the operator can only actuate either the "fullpower" or "variable power" switch after a "residual" or "release"function has been performed. Actuation of the "release" function switchis prevented when the residual function switch is actuated.

The digital control circuit 26 includes an oscillator or digital timer,such as a commercially available No. 556 dual timer, which contains two555 timers designated at 128a and 128b. Timer 128a constitutes a stableoscillator which has its frequency determined by a resistor 130, arelease cycle timing adjustment 132, a resistor 134 and a capacitor 136which are connected to form an R-C network coupled to the oscillator128a at its pin connections 1, 2 and 6, as illustrated in FIG. 4. Therelease cycle timing adjustment 132 enables factory or on-siteadjustment of the rate at which the polarity of the output voltagesignals applied to chuck 12 by power output circuit 24 are reversedduring a demagnetizing operation, and also the rate at which the outputacross the chuck will decay to zero VDC. For example, the time releasecycle during demagnetizing might be selected as approximately 5, 10 or20 seconds.

The timer 128b constitutes a monostable oscillator circuit which istriggered at pin 8 by a capacitor 138 charging through a resistor 140 toVCC. This begins when power is first supplied to the primary oftransformer 72. At this first instant of time, capacitor 138 appears asa short to ground, causing pin 8 to appear grounded. This causes theoutput pin 9 to go high. Pin 9 will remain high ("1") until a capacitor142 charges to two-thirds of VCC through a resistor 144 at which timepin 9 goes to ground. The output at pin 9 of timer 128b is directed topin 14 of a BCD up/down counter 148 which clears the counter. The outputat pin 9 of timer 128b is also directed through an inverter 150 (pin 6being "0") to a dual J-K flip flop 152a,b (pins 2 and 7, respectively)so as to set the outputs of pins 14 and 10, respectively, at "0". Pin 6of inverter 150 returns to a "1" state when pin 9 of timer 128b goes toground ("0").

With power applied to the primary of transformer 72, output pin 13 ofcounter 148 is at a "0" directed through an inverter 154 so that its pin11 is at "1". Pin 4 of oscillator 128a is at "1" and output pin 5oscillates back and forth from "0" to "1" to "0", etc. Pin 2 of a NANDgate 156 is "0" so its output pin 3 is "1". With pins 9 and 10 of a NANDgate 158 at "1", its output pin 8 is "0". This enables the green"Release " function indicator LED 124 to light. LED indicators 118, 120and 122 remain off. The "0" signal on pin 8 of NAND gate 158 is alsodirected to a display driver 160 (pin 5) which blanks out the display 28so that it shows no numerical or polarity symbols. At this time, thecircuit enables either full power or variable power function to beselected and disables residual power and release functions.

As aforementioned, full power can only be ohtained from the residual orrelease state or condition of the control circuit 10. With reference tothe digital control circuit 26 shown in FIG. 4, full DC power to thechuck 12 is obtained by depressing switch 110 which pulls pin 1 of aNAND gate 164 to the "0" output or ground of pin 6 of a NAND gate 166.Simultaneously, pin 6 of a NAND gate 168 goes to "0" and indicator LED118 turns on indicating "full power " on. Pin 12 of a NAND gate 170 goesto a "0". Pin 8 of NAND gate 176 goes to "0", stopping the oscillator ortimer 28a with its pin 4 going to "0" and its pin 5 going to "1".

Pin 4 of oscillator 128a is connected to a divider in the form of J-Kflip flops 178a,b (with clear only) such that the clear inputs to pin 2of flip-flop 178a and pin 6 of flip-flop 178b go to "0" which resets thedivider forcing pin 9 to "0", pin 8 to "1", pin 12 to "0" and pin 13 to"1". These signals from flip-flop 178a force pin 3 of a NAND gate 182and pin 8 of a NAND gate 184 to "0". The signal is fed from pin 8 ofNAND gate 184 through a signal time delay circuit which includes amonostable multivibrator 186, an inverter NAND gate 188 and the J-Kflip-flop 152b where the "0" appears at the output pin 10 after apredetermined delay and is directed to optical coupler 106a through aconductor 190. Pin 3 of NAND gate 182 goes directly to the opticalcoupler 106c through conductor 192. Thus, optical couplers 106a, 106cand the associated triacs 104a and 104c are turned on.

Turning on switch 110 also causes pin 11 of counter 148 to "0" throughan inverter 196 causing the counter to preset to a predeterminednumerical value as determined by pins 1, 9, 10, and 15. This numberdetermines the quantity of pulses applied to the chuck during a releaseor demagnetizing cycle and is encoded on pins 2, 3, 6 and 7 of thecounter. This code is fed to pins 1, 2, 6 and 7 of the LEDdecoder/driver 160 which decodes the code signal and disolays it as adecreasing decimal number on the digital display 28 during a releasecycle. At this time a "0" on pin 3 of an inverter 200 is directed to pin4 of the decoder to blank out the digit loaded into pin 5 of counter 148so that display 28 only shows a (+) polarity sign. Simultaneously, pin13 of counter 148 is forced to "1" so that pins 12 and 13 of inverter154 are "1", and inverted pin 11 is "0". Pin 8 of NAND gate 158 is "1".This turns off the release indicator LED 124. At this time, pin 10 of aNAND gate 202 is "1" and pin 9 is "1" so that its output pin 8 is "0"and is inverted through an inverter 204 to "1" at pin 11. This disablesor locks out the variable power switch 112 so that only residual powerswitch 114 or release switch 116 will function.

The digital display 28 may comprise a seven segment LED display havingplus and minus polarity indication. The display 28 is adapted to displayas a digital countdown the number of cycles or steps of reverse polaritysuccessively reduced voltage pulses applied to the magnetic chuck 12during a demagnetizing or release operation and also indicates thepolarity of the particular pulse being applied. In the describedembodiment, the display 28 will display the number "9" when ademagnetizing cycle is initiated and will countdown until the displayreads "0" at which time the voltage potential at the chuck is zero VDCand the demagnetizing or release sequence is completed. The counter 148is preferably set so that 10 pulses are established over a time periodof approximately 7-8 seconds.

The number encoded on output pins 2, 3, 6 and 7 of counter 148 is fed topins 10, 11, 12 and 13 of a digital-to-analog converter 208 whichmultiplies the encoded number from the counter by a reference voltage aspresent at pin 15 of the converter 208. In the described embodiment, thereference voltage is derived from pin 16 of converter 208 which is a (+)2.5 VDC precision reference voltage connected to a variable DC outputadjustment 210. The setting of variable adjustment 210 determines themultiplier at pin 15 of converter 208 and controls the analog outputvoltage at pin 14 of the converter. The variable adjustment 210 includesan external control knob 210a mounted on the panel 36 to enable operatorselection of a variable voltage less than 100% of the full power voltageto be applied to the magnet 12 during a variable voltage mode ofoperation.

In the case of full power selection, a "0" appears on pin 1 of a NANDgate 212 when the full power switch 110 is closed. This makes a "0"appear at pin 6 of a NAND gate 214 and at pin 2 of an optically isolatedtransistor coupler 218, thus turning this coupler on. The output of pins4 and 5 of coupler 218 effectively short out pins 15, 16 of converter208. This is like setting the variable power adjustment 210 to the 100%setting. The 0% setting of adjustment 210 is connected through a diode220 to pin 3 of NAND gate 212 which is now sitting at a "1" level,effectively removing adjustment 210 from the digital control circuit.The output at pin 14 of converter 208 is maximum at this time andadjusting the variable adjustment 210 has no effect on the DC outputwhich is maximum. During selection of variable power or releasefunctions, optical coupler 218 is off, returning variable poweradjustment 210 to its normal function.

The analog output from pin 14 of converter 208 is fed through aconductor 222 to pin 3 of an operational amplifier 224 and amplified toa sufficient level required by a phase control network 226. The controlvoltage input across pins 8(+) and 6(-) of the phase control 226determines the phase angle at which a control triac 230 will fire. Zerovolts between pins 8 and 6 of phase control 226 will make triac 230conduct fully. Increasing the voltage at pin 8 with respect to pin 6 ofphase control 226 will make triac 230 conduct less. The operationalamplifier 224 is thus connected as an inverting amplifier; that is, witha maximum voltaqe input to pin 3, the output at pin 4 is zero and viceversa. An optical coupler 232 controls the state of triac 230. Withcoupler 232 turned off, triac 230 through phase control 226 is alwaysoff. A low voltage adjustment 234 is connected to the phase controlnetwork 226 to enable adjustment as necessary to insure zero voltageacross the chuck terminals 96a,b when the variable output adjustment 210is set at zero percent of full power.

The optical coupler 232 is controlled by the "0" on pin 9 of flip-flop178b directed through a signal time delay circuit made up of amonostable multivibrator 236, an inverter 238 and a J-K flip-flop 240the output pin 13 of which is at "0" after a predetermined time delay.The "0" state from pin 13 of flip-flop 240 is directed to pin 5 of aNAND gate 242. Pins 9 and 10 of NAND gate 244 are "1" making pin 8 "0"and pin 2 of optical coupler 232 "0" so as to turn it on.

With the triacs 104a and 104c having previously been turned on byclosing switch 110 as aforedescribed, and with full power being appliedto amplifier 224, triac 230 will conduct fully so that the maximumoutput voltage from bridge rectifier 92 is applied to the magnetic chuck12 to maintain the ferromagnetic work piece or object 14 insubstantially fixed relation on the magnetic chuck irrespective of thesetting of the variable DC output adjustment 210.

Variable power operation can only be obtained from the residual orrelease states. Variable DC power to the magnetic chuck 12 is obtainedby depressing switch 112 which pulls pin 13 of a NAND gate 248 to the"0" output or ground of pin 11 of inverter 204. Simultaneously, pin 8 ofa NAND gate 250 goes to "0" and indicator LED 120 turns on. Theremaining events which take place in the variable DC mode of operationare the same as in the aforedescribed full DC power mode of operationwith two exceptions: Firstly, the "0" state on pin 5 of NAND gate 214causes its output pin 6 to be "1". Thus, optical coupling 218 is off andvariable power adjustment 210 is operative. Secondly, when indicator LED124 is turned off by the "1" level appearing on pin 8 of NAND gate 158,pin 1 of a NAND gate 252 is a "1" and pin 2 is a "1" so that output pin3 is "0" and is inverted through inverter 166 at pin 6 to "1". Thisdisables the full power switch 110 so that only residual power switch114 and release switch 116 will function. In this manner, the full powerswitch 110 is digitally interlocked so as to prevent its closing duringa variable power mode of operation.

The residual power mode of operation can only be obtained after a fullpower or variable power state has been obtained and not during a releasemode. Residual power to the chuck 12 is obtained by depressing switch114 which pulls pin 5 of a NAND gate 256 to the "0" output of pin 13 ofNAND qate 174. Simultaneously, pin 6 of NAND gate 256 goes to "1" and isinverted through an inverter 258 so that its pin 8 is "0". This turns onindicator LED 122 and disables release switch 116. At this time eitherindicator LED 118 or LED 120 (not both) is on. Pin 1 of a NAND qate 260is at "1", forcing pin 3 to "0". Pin 9 of a NAND gate 244 is "0" and pin10 is "1" so as to force its output pin 8 to "1" which appears at pin 2of optical coupler 232 and shuts it off. With coupler 232 off, triac 230is shut off and the DC output voltage across chuck terminals 96a and 96bgoes abruptly to zero.

With the residual mode now reached, only one function switch, eitherfull power switch 110 or variable power switch 112, will reset thecircuit, depending whether indicator 118 LED or LED 120 is on. Forexample, if LED 118 is on, pin 2 of NAND gate 252 is "0" and pin 1 is"1". Pin 3 is "1" and through inverter 166 pin 6 is "0". This enablesfull power switch 110. With LED 120 off, pin 9 of NAND gate 202 is "1",pin 10 is "1" and pin 8 is "0" so that pin 11 of inverter 204 is "1".This disables variable power switch 112. The reverse is true if LED 120is on and LED 118 is off, i.e., switch 112 would be enabled and switch110 would be disabled.

With full power switch 110 enabled as in the above example, depressingswitch 110 puts pin 12 of NAND gate 170 at "0" and pin 13 at "1" so thatpin 11 is "1". With pins 9 and 10 of NAND gate 172 at "1", output pin 8is "0" so that pin 1 of a NAND gate 262 is "0". This resets pin 6 ofNAND gate 256 to "0" so that pin 8 of inverter 258 is "1". Indicator LED122 is thereby turned off. Release switch 116 is enabled and pin 1 ofNAND gate 260 is "0" making pin 3 "1". Pins 9 and 10 of NAND gate 244are "1" and output pin 8 is "0" which is directed to pin 2 of opticalcoupler 232. This turns coupler 232 on and returns the digital controlcircuit back to the full or variable power state or mode.

After performing one or more desired operations on the work piece 14,the release or demagnetizing mode of operation of the magnetic chuckcontrol system 10 is initiated by closing release switch 116 such as bypressing the sealed membrane switch 116 mounted on the control panel 26.As aforementioned, the release state can only be obtained after a fullpower or variable power mode has been obtained, but not after a residualpower mode of operation.

Depressing the release switch 116 to initiate a release mode or cyclepulls pin 1 of a NAND gate 266 to the "0" output or ground of pin 6 ofNAND gate 256. Simultaneously, a "0" on pin 1 of NAND gate 266, pin 2being at "1", forces output pin 3 to "1" such that pin 6 of an inverter268 is "0" and pin 9 of NAND gate 176 is "0". This makes pin 8 of NANDgate 176 go to "1" which is directed to pin 4 of oscillator timer 128aand causes it to oscillate. The "0" on pin 1 of NAND gate 266 is fedback to pin 5 of NAND gate 168 and pin 9 of NAND gate 250. Whicheverindicator LED (LED 118 or LED 120) is on will be turned off.

Depressing the release switch 116 also causes the decimal numberrepresenting the preset value of the encoded number on counter 148 to bedigitally displayed on display 28. At this time, pin 4 of timer 128a,pin 4 of decoder/driver 160, and pins 11 and 13 of counter 148 are "1".The counter 148 begins to count down oscillator pulses applied at itspin 4 from the output pin 5 of timer 128a through NAND gate 156 and J-Kflip-flop 178b. The pulses from pin 8 of J-K flip-flop 178b are also fedthrough J-K flip-flop 178a and two inverters 270 and 272 to NAND gates182 and 274. The inverters 270 and 272 do not change the state of thepulse applied from input pins 12 and 13 of NAND gate 270 to output pin 8of NAND gate 272 but cause a predetermined delay which enables thearrival of all signals at NAND gates 182 and 274 to be simultaneous. Pin3 of NAND gate 182 and pin 8 of inverter 184 are now at "1", while pin 6of an inverter 276 and pin 11 of NAND gate 274 are at "0".

The "0" signal from pin 6 of inverter 276 is fed through a signal timedelay circuit made up of a monostable multivibrator 278, an inverter 280and the J-K flip-flop 152a where the "0" appears at the output pin 14after a predetermined delay and is directed to pin 2 of optical coupler106b through a conductor 282. The "0" signal from pin 11 of NAND gate274 is fed directly to optical coupler 106d through a conductor 284.This turns optical couplers 106b, 106d and corresponding triacs 104b,104d on which reverses the polarity of DC output voltage at terminals96a and 96b of the chuck 12 and decreases its magnitude by a valuedefined as the quotient of the original DC voltage applied at the outsetof the release cycle divided by the total number of pulses during therelease cycle. For example, if 115 VDC is being applied to the chuck 12when a release cycle is initiated, and the timer 128a is set toestablish ten pulses during a release cycle, then the output voltageapplied by bridge rectifier 92 to the chuck decreases by a value of 11.5VDC during each polarity reversal during the release cycle. Thisreversing of polarity and stepped decreasing of applied voltage to thechuck continues until counter 148 reaches a number zero. At this timethe DC output voltage at terminals 96a,b of the chuck will be zero, andthis is indicated by the display 28 visually displaying a "0". A " 0"signal from pin 13 of counter 148 is transmitted to pins 12 and 13 ofinverter 154 and to pin 2 of NAND gate 156 so that pin 8 of NAND gate158 goes to "0". This makes indicator LED 124 turn on indicating therelease cycle is complete.

During the release cycle the following takes place at the terminals 96aand 96b of magnetic chuck 12. A DC voltage appears as a "+" polarity at96a and a "-" polarity at 96b through triacs 104a and 104c. The opticalcoupler 106a is turned on with the output pin 10 of J-K flip flop 152bat "0", and coupler 106c is turned off when the output pin 3 of NANDgate 182 goes to "1". The phase control triac 230 is turned off throughthe phase control network 226 and pin 2 of optical coupler 232 going to"1" from NAND gate 244. The triacs 104a and 104c remain in a conductingstate or "on" condition from the stored energy in the inductive load ofthe chuck and work piece which maintains the voltage across triacs 104aand 104c.

With optical coupler 106a on, optical coupler 106b is turned on by pin 6of inverter 276 going to "0" momentarily. The timing sequence isgoverned by the J-K flip flops 178a and 178b. The "0" signal from pin 6of inverter 276 is delayed a preselected amount of time, such asapproximately 20 milliseconds, by the delay circuit formed bymultivibrator 278, inverter 280 and J-K flip flop 152a.

Turning on the optical couplers 106a and 106b turns on the correspondingtriacs 104a and 104b forming a crow-bar short across the chuck terminals96a,b so as to dissipate the stored energy in the inductive load. Asthis energy is dissipated, the voltage across the two triacs 104a and104b goes to zero at which time triacs 104a,b turn off.

Substantially simultaneously with turning off the two triacs 104a and104b, triacs 104b and 104d turn on and triac 230 turns on after a shorttime delay, such as approximately 300 milliseconds, as established bythe time delay circuit of multivibrator 236, inverter 238 and J-K flipflop 240. A DC voltage signal appears at the chuck terminals 96a,b whichhas been decreased a predetermined stepped amount and is of oppositepolarity from the immediately preceeding voltage signal applied to thechuck.

The optical coupler 106b remains on from the output pin 14 of J-K flipflop 152a being at "0". Triac 230 and optical couplers 232 and 106d arethen turned off. Triacs 104b and 104d remain on from the stored energyin the load comprising the chuck and work piece. Optical coupler 106a isagain turned on momentarily in timed sequence governed by the J-K flipflops 178a and 178b. The "0" signal from inverter 184 is delayed apreselected time period, such as approximately 20 milliseconds, throughthe time delay circuit of multivibrator 186, inverter 188 and J-K flipflop 152b. After a short time, triac 104a turns on. Triac 104b is on soas to form a crow-bar short across the output terminals 96a,b at thechuck 12 to again dissipate the stored energy in the inductive load.This process repeats itself until the counter 148 reaches the numberzero at which point the release cycle is complete as indicated byturning on of the LED 124 and the blanked out digital display 28. Bybeing digitally controlled, the successive voltage reductions to thechuck are linear; that is, the voltage reductions are in precise equalsteps. The final voltage applied to the chuck is substantially zero.

FIGS. 5 and 6 illustrate, respectively, a logic diagram and a schematicdiagram illustrating the optical couplers 106a,b,c and d as controlledby the J-K flip flops 178a and 178b. The logic diagram of FIG. 5indicates the various logic states, i.e. "0" or "1", of the various pinconnections of the J-K flip flops 178a and 178b, in relation to thecorresponding current conduction and polarity states of the magneticchuck 12. FIG. 6 indicates the logic states, i.e. "0" or "1" , of theNAND gates 182 and 274 and inverters 184 and 276, and the correspondingstates of the optical couplers 106a,b,c and d. A "0" on pins 2 of theoptical couplers 106a,b,c and d turns them on while a "1" signal turnsthem off.

The various described time delays for signals applied to the opticalcouplers 106a, 106b and 232 and thereby to the triacs 104a, 104b and 230delay the turn on of these elements but not their turn off. The varioussignal time delays prevent simultaneous turning on and off of theassociated elements.

Thus, in accordance with the present invention, a system for controllinga magnetic chuck is provided which employs a completely digital typecontrol circuit portion which is substantially immune to noise andambient temperature variations such that drifts and erratic operationare eliminated. By employing a relatively low 5 VDC power supply for allcircuits except the power output circuit, and by isolating the poweroutput circuit from the remaining circuits through optical couplers, thehighest potential that might be realized at the control panel switchesin the event of a fault current in the power output circuit would be aharmless 5 VDC. Further, by employing triacs and associated opticalcouplers in the reversing circuit 100, the control circuitry isprotected from any power surges at the output of the power outputcircuit 24 at all times, thus assuring elimination of any unexpected orerratic outputs.

In applications where it is desired that the holding or power outputvoltage of the power output circuit 24 be less than full power, i.e.approximately 115 VDC, the only adjustment required is to decrease thereference voltage through adjustment of variable adjustment knob 210a,thus making its product with the value encoded on the countercorrespondingly smaller.

While a preferred embodiment of the present invention has beenillustrated and described, it will be understood that changes andmodifications may be made therein without departing from the inventionin its broader aspects. Various features of the invention are defined inthe following claims.

What is claimed is:
 1. A system for magnetizing and demagnetizing amagnetizable object, comprising, in combination,a DC power supplyincluding a power supply circuit having an AC power supply and a bridgerectifier adapted to establish said DC power supply, a power outputcircuit operatively connected to said DC power supply and a magnetizableobject and adapted to apply DC voltage signals to the object in a mannerto magnetize the object, said output circuit including a reversingcircuit enabling reversing of the polarity of said DC voltage signalsduring demagnetizing, said reversing circuit defining a pair of inputterminals connected in circuit with said DC power supply and a pair ofoutput terminals connected to the object, bidirectional current controlmeans selectively connected between said pairs of terminals so as toenable selective directional current flow through the object, a digitalcontrol circuit operatively associated with said current control meansand operative to effect predetermined sequential conditioning of saidbidirectional current control means so as to enable current flow throughthe object in alternating directions, a digital-to-analog convertercircuit cooperative with said digital control circuit and adapted toproduce successive output voltage signals of predetermined decreasingmagnitude in direct relation to said predetermined conditioning of saidbidirectional control means, and means connected in circuit with said DCpower supply and said reversing circuit and being responsive to saidsuccessive output signals to control successively decreasing DC voltagesignals to the object, said digital control circuit being adapted tocontrol said bidirectional current control means so that saidsuccessively decreasing DC voltage signals applied to the object are ofalternating polarity, said means for controlling said successivelydecreasing DC voltage signals to said object including switch meansconnected in said power supply circuit between said AC power supply andsaid bridge rectifier and being controlled by said digital-to-analogconverter circuit and said digital control circuit.
 2. The system asdefined in claim 1 wherein said switch means includes a triac controlledby said digital control circuit.
 3. A system for magnetizing anddemagnetizing a magnetizable object, comprising, in combination,a DCpower supply, a power output circuit operatively connected to said DCpower supply and a magnetizable object and adapted to apply DC voltagesignals to the object in a manner to magnetize the object, said outputcircuit including a reversing circuit enabling reversing of the polarityof said DC voltage signals during demagnetizing, said reversing circuitdefining a pair of input terminals connected in circuit with said DCpower supply and a pair of output terminals connected to the object,bidirectional current control means selectively connected between saidpairs of terminals so as to enable selective directional current flowthrough the object, a digital control circuit operatively associatedwith said current control means and operative to effect predeterminedsequential conditioning of said bidirectional current control means soas to enable current flow through the object in alternating directions,a digital-to-analog converter circuit cooperative with said digitalcontrol circuit and adapted to produce successive output voltage signalsof predetermined decreasing magnitude in direct relation to saidpredetermined conditioning of said bidirectional control means, andmeans connected in circuit with said DC power supply and said reversingcircuit and being responsive to said successive output signals tocontrol successively decreasing DC voltage signals to the object, saiddigital control circuit being adapted to control said bidirectionalcurrent control means so that said successively decreasing DC voltagesignals applied to the object are of alternating polarity, said systemincluding full power, variable power, residual and release modes ofoperation, and said digital control circuit including means preventinginitiation of a full power mode of operation when the system is in avariable power mode of operation.
 4. The system as defined in claim 3wherein said digital control circuit includes digital control meanspreventing initiatin of a residual power mode of operation when thesystem is conditioned in a release mode of operation.
 5. The system asdefined in claim 3 wherein said digital control circuit includes digitalcontrol means preventing initiation of a release mode of operation whenthe system is conditioned for a residual power mode of operation.
 6. Asystem for magnetizing and demagnetizing a magnetizable object,comprising, in combination,a DC power supply, power output circuitoperatively connected to said DC power supply and a magnetizable objectand adapted to apply DC voltage signals to the object in a manner tomagnetize the object, said output circuit including a reversing circuitenabling reversing of the polarity of said DC voltage signals duringdemagnetizing, said reversing circuit defining a pair of input terminalsconnected in circuit with said DC power supply and a pair of outputterminals connected to the object, bidirectional current control meansselectively connected between said pairs of terminals so as to enableselective directional current flow through the object, a digital controlcircuit operatively associated with said current control means andoperative to effect predetermined sequential conditioning of saidbidirectional current control means so as to enable current flow throughthe object in alternating directions, a digital-to-analog convertercircuit cooperative with said digital control circuit and adapted toproduce successive output voltage signals of predetermined decreasingmagnitude in direct relation to said predetermined conditioning of saidbidirectional control means, and means connected in circuit with said DCpower supply and said reversing circuit and being responsive to saidsuccessive output signals to control successively decreasing DC voltagesignals to the object, said digital control circuit being adapted tocontrol said bidirectional current control means so that saidsuccessively decreasing DC voltage signals applied to the object are ofalternating polarity, said digital control circuit being operative toproduce discrete digital control signals of predetermined time duration,and including means responsive to said discrete control signals forselectively conditioning said bidirectional current control means forcurrent flow therethrough, said reversing circuit comprising a bridgetype network having bridge legs connected between alternate ones of saidinput and output terminals, each of said bridge legs having a triacconnected in circuit therein, said digital control means beingoperatively connected to said triacs so as to enable selective currentflow therethrough in direct response to said discrete digital controlsignals, said triacs having their gates operatively connected to saiddigital control circuit so that said triacs may be selectivelycontrolled by a relatively low supply voltage applied to said digitalcontrol circuit, the gate of each of said triacs being interconnected tosaid digital control circuit through a corresponding transistorcontrolled opto-coupler.
 7. A system for magnetizing and demagnetizing amagnetizable object, comprising, in combination,a DC power supply, apower output circuit operatively connected to said DC power supply and amagnetizable object and adapted to apply DC voltage signals to theobject in a manner to magnetize the object, said output circuitincluding a reversing circuit enabling reversing of the polarity of saidDC voltage signals during demagnetizing, said reversing circuit defininga pair of input terminals connected in circuit with said DC power supplyand a pair of output terminals connected to the object, bidirectionalcurrent control means selectively connected between said pairs ofterminals so as to enable selective directional current flow through theobject, a digital control circuit operatively associated with saidcurrent control means and operative to effect predetermined sequentialconditioning of said bidirectional current control means so as to enablecurrent flow through the object in alternating directions, adigital-to-analog converter circuit cooperative with said digitalcontrol circuit and adapted to produce successive output voltage signalsof predetermined decreasing magnitude in direct relation to saidpredetermined conditioning of said bidirectional control means, andmeans connected in circuit with said DC power supply and said reversingcircuit and being responsive to said successive output signals tocontrol successively decreasing DC voltage signals to the object, saiddigital control circuit being adapted to control said bidirectionalcurrent control means so that said successively decreasing DC voltagesignals applied to the object are of alternating polarity, said digitalcontrol circuit being operative to produce discrete digital controlsignals of predetermined time duration, and including means responsiveto said discrete control signals for selectively conditioning saidbidirectional current control means for current flow therethrough, andsaid digital control circuit including time delay circuit meansoperative to delay application of selected ones of said discrete digitalcontrol signals to said bidirectional current controls means.